High frequency coupling system



July 6, 1937. A. H. TURNER 2,086,342

HIGH FREQUENCY COUPLING SYSTEM Filed Sept. 29, 1934 I/v VENTOBJ @M KQ lbetween the coupled circuits.

Patented July 6, 1937 UNITED STATES 2&85342 HIGH FREQUENCY oocrrmo. SYSTEM Delaware Application September 29, 1934, Serial No. 746,097

7 Claims.

The present invention relates to coupling systerms for tuned high frequency circuits and, more particularly, it relates to coupling systems for tuned ultrahigh frequency circuits for television and other-high frequency radio apparatus. Such systems may operate in the range of 40 to 80 megacycles, for example.

It is a primary object of the present invention to provide a system for coupling two or more lO-high frequency tuned circuits of the above character to permit such circuits to be tuned through a predetermined range with substantially constant band width throughout the range, and to provide substantially constant voltage transfer However, my invention provides a considerable flexibility of control of the degree of coupling over any portion of the tuning range without altering the degree of coupling over the rest of the tuning range.

Constant band width-between the frequencies of 90% of maximum response requires that the coupled circuitsbe over-critically coupled at the low frequency end of the'tuning range and under-critically coupled at the high frequency end.

5 Ordinary fixed combinations of inductive and capacitive coupling are not capable of providing this desired variation of coupling with tuning.

In accordance with the invention, two or more coupled circuits are tuned by variable tuning means having a conductive shaft common to said circuits. The variable tuning means may comprise a gang-type variable capacitor having a common rotor and shaft, and spaced stator elements supported along the shaft in a bath tub frame and insulated therefrom; with-the'common conductive shaft journaled in the ends of the frame.

denser, are mounted on the frame. 40 At very high frequencies the common rotor shaft presents appreciable impedance to the currents flowing through it. The potentials appearing on the rotor shaft by virtue of these currents from different circuits cause coupling between these circuits. Experiments have been made to determine the relative magnitude and direction of the coupling which occurs in the rotor shaft of the common tuning means. I have found that the rotor currents through the shaft produce a high value of coupling between circuits common to the shaft in the low frequency end of the tuning range, whereas at the high frequency end of the tuning range comparatively slight coupling through the rotor shaft exists b cause a large part of the total circuit currents 'wiring.

The term inductive coupling is herein con- 7 Suitable trimmer capacitors for the various tuned circuits associated with the conno longer flows through the rotor shaft but follows other paths to the capacitor frame. These other paths may be through the vacuum tube inter-electrode capacity, trimmer capacitorcapacity and the distributed capacity of circuit sidered to be that produced bythe mutual-inductance of two or more coils; conductive coupling refers to that produced by the self inductor common to two or more circuits.

The potentials onthe rotorshaft are largely reactive and are therefore approximatelyin phase or 180 out of phase withpotentials furnished by inductive coupling. However, in the case of capacitive coupling no control'is had overthe relative phasing. and italways must oppose the conductive coupling of the rotor shaft. Taken separately, the conductive coupling: increases with decreasing-frequency, the inductive coupling may remain-approximately constant, and thecapacitive couplingincreases with increasing frequency.

frequency circuits wherebythe conductive cou pling through the shaft of a common tuning.

PATENT osrics I means-may be adjusted andcontrolledto so com-w;

bine with the inductive and-stray capacitive coupling that the overall coupling between ;the various tuned circuits may be-causedto vary at such I arate tosproduce constant voltage across one ofthe circuits and a substantially constant band widthof response throughout the tuning range;

Inasuperheterodyne receiver tuning-over the range of 40to BOmegacycles, the desired conditions of approximately constant bandwidth at response are obtained by using largely con ductive coupling between two signal frequency circuits with slight inductive coupling aiding and slight capacitive coupling opposing. The oscillator is coupled similarly to one of the signalfre quency circuits. At the low frequency end of the tuning range the intermediate frequency, which is the difference between the oscillator and signal frequencies, is a larger percentage'of the signal frequency; Therefore, the oscillator voltage supplied to'the grid of the first detector. would be remained constant. keep constant oscillator voltage on the grid of the first detector and this is readily done by using conductive coupling which increases with decreasing frequency. This form of coupling can even be made to compensate for the usual decrease of oscillator amplitude at the low frequency end of the tuning range.

It is, therefore, a still further object of the invention to provide a coupling system for a plurality of high frequency circuits, tuned to the same or to different frequencies, which automatically provides for increasing the coupling between the circuits at the low frequency end of the tuning range to maintain the voltage transfer substantially constant.

The magnitude of the conductive coupling may be regulated by selecting the proper size and length of shaft or by regulating the reactance of the shaft between the various sections of the tuning means, such as between the sections of a gang tuning capacitor or between certain or all of the sections and the ground or frame of the capacitor as by contact brushes mounted on the frame and bearing on the shaft.

The invention will, however, be understood better from the following description, when considered in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

In the drawing,

Figs. 1 and 2 are top and side views, respectively, of a high frequency tuning unit embodying the in vention, Fig. 2 being partly in section;

Fig. 3 is a schematic circuit diagram of the tuning unit of Figs. 1 and 2 showing the distribution of high frequency currents in the shaft of the common tuning element thereof; and

Fig. 4 is a schematic circuit diagram similar to that of Fig. 3 illustrating further modifications of the invention.

Referring to Figs. 1 and 2, I0 is the frame of a bath tub tuning capacitor of the gang type comprising a common shaft ll' journaled in the ends of the frame and carrying a plurality of rotor sections or elements [2, l3, and I4, each being associated with a fixed stator section or element l5 (Fig. 2). The rotor elements are secured to the shaft while the stator elements are supported on brackets l 6, l I insulated from the frame. The bracket l1 provides a metallic connection with a spring leaf plate l8 and a terminal I9 of a trimmer capacitor, the opposite plate of which is the frame. A piece of mica or other insulating material 20 serves to separate the capacitor plate l8 from the frame, and an adjusting nut 2| is provided for adjusting the plate I 8 in the direction of the frame to vary the capacity of the trimmer capacitor.

Since the rotor of the capacitor is grounded to the frame, the trimmer capacitors are substantially in parallel with the main capacitor units or sections, and serve to tune a series of coupled circuits, each comprising a section of the capacitor and a tuning inductor. In the present example, the tuning inductors are arranged for tuning response over a frequency range of 40 to megacycles with a capacitor of the type shown, and comprise coils having a relatively few number of turns and of small diameter coils 22, 23, 24, wound upon suitable forms 25 mounted adjacent to the terminals IQ of the trimmer capacitors for each section. In order that the coil leads may be relatively short as shown it will be noted that one lead of each coil is connected to the adjacent trimmer capacitor terminals l8 While the opposite lead is grounded to the frame, as indicated,

for example, at 26, also adjacent to each coil.

It will be noted that the inductance coils or windings 22, 23, and 24 are arranged in inductively coupled relation to each other, and are the an-' tenna, detector and oscillator coils, respectively, for a superheterodyne receiver. Each of the three tuned, magnetically or inductively coupled, circuits thereby provided, comprises a variable capacitor section, an inductor and a trimmer capacitor in parallel, the common tuning device or capacitor having a shaft adapted to provide conductive coupling between the circuits. It will also be noted that the capacitor shaft H is relatively small in diameter. This is for the purpose of providing a desired inductive reactance for coupling purposes between the various circuits. Stray capacitive coupling is also present between the various leads and between the capacitor stator elements.

The arrangement of the parts as shown in Figs. 1 and 2 are shown, described, and claimed in the copending application of Ralph S. Holmes, Serial No. 708,547, filed January 27, 1934, and assigned to the same assignee as this application. As the physical relation of the parts of the apparatus shown does not concern the present invention further description is believed to be unnecessary.

Referring now to Fig. 3, in which the same reference numerals refer to the same parts as in Figs. 1 and 2, it will be seen that the high potential side, or trimmer terminal connection for each circuit is connected with tube electrodes or other sources of capacity as schematically represented by the antenna circuit 30 connected through a coupling capacitor 3| with the inductor 22, a detector 32 having a grid connection with the coil 23, and an oscillator 33 having a grid connection with the coil 24. The circuits are completed through ground connections to the capacitor frame H).

In the diagram, the predominating paths of the radio frequency currents at the low frequency end of the tuning range are indicated by the arrows 35 along the shaft and the predominating paths of the high frequency currents at the high frequency end of the tuning range are indicated by the curve arrows 36 associated with each trimmer capacitor. From an inspection of Fig. 3 it will be seen that the currents shift from the common coupling impedance of the shaft to the distributed capacity of the various circuits at the high frequency end of the tuning range, provided principally by the trimmer capacitors, thereby decreasing the coupling between the circuits.

The degree of coupling at the low frequency end may be regulated by choosing a shaft of the proper diameter and length or a shaft of a given diameter may be reduced between the capacitor sections as indicated by the dotted lines 31, the reduction in diameter of the shaft causing the inductive reactance thereof to increase. The trimmer capacitor l8 may be taken to also represent any distributed capacity across the circuit such as the inter-electrode capacity of the tubes 32 and 33 or the antenna capacity.

Referring now to Fig, 4, a 3 unit tuning system is represented wherein main tuning sections are indicated at 40, 4|, and 42 in association with the rotor element comprising a shaft 43 supported in a frame 44 to which are applied trimmer capacitors 45 in connection with tuned circuits having inductors 46, the tuned circuit arrangement being similar to that of Fig. 3 except that the shaft is provided with insulating sections 41,

48, and 49 at spaced intervals or between the sections and the frame to prevent or reduce the flow of high frequency currents through the shaft. The shaft may also further be provided with a reduced diameter section 50 for the same purpose.

Variable reactance or impedance elements of any suitable type, such as inductors may be connected in shunt around the insulating sections and reduced sections of the shaft as indicated at i and 52 to control the current flow through the shaft. Likewise variable resistances as indicated at 53 and 54 may be provided for the same purpose. By. varying the impedance at each of the points indicated, the flow of current throughthe shaft and between the sections may be regulated as desired to produce the desired degree of coupling at the low frequency end of the tuning range.

Furthermore the coupling may be regulated by impedances between any one, or all, of the sections to ground or frame as indicated by the variable resistance connection 55 between the capacitor section 49 and frame. It will be noted that in the case of the modification shown, conthe arrow points 56. This necessitates the use of brushes or other contact making means to establish the connection between the external control element and the shaft sections. It is preferable, however, to resort to a proper design in shaft size or a reduction or increase inthe crosssection along the shaft in order to regulate the flow of current through the shaft, and the low frequency coupling, without resorting to brush contacts.

Sincethe inductive and/or capacitive coupling is a matter of position and direction of winding of the coils, the conductive coupling through the shaft may thus be controlled and may be made to aid or oppose the said coupling, and both the inductive and conductive coupling may be made to oppose the stray or distributed capacity coupling between the various circuits. 7

While three tuned coupled circuits have been shown that are adapted primarily for coupling the oscillator, detector and signal input circuits of a superheterodyne receiver, the system is adapted for coupling any of a plurality of tunable circuits to obtain a uniform response or voltage transfer over a relatively wide frequency range and a uniform frequency band width throughout that range. To this end, the conductive coupling through the shaft is ordinarily employed to increase the coupling in aiding relation to the inductive coupling at the low frequency end of the tuning range where the circuits are ordinarily more sharply tuned and require over-coupling.

It is, of course, to be understood that the distributed capacity coupling between adjacent circuits is out ofphase with the conductive,

coupling and therefore opposes the conductive coupling principally at the high frequency end of the tuning range. Likewise, distributed capacity of either of the circuits to ground reduces the percentage, in the rotor shaft, of the total circuit current. Therefore, increasing any one of these capacities increases the rate at which the coupling increases with decreasing frequency. It also often happens that the capacity coupling is greater at the high frequency end than the conductive coupling (assuming inductive coupling to be negligible), in which case the resultant coupling will be zero somewhere within the tuning range.

From the foregoing description, it will be seen that the variable tuning means includes a shaft of a tuning device common to the variouscoupled circuits, andthat the distributed capacity of the associated circuits, as provided by the leads and circuit elements, predominates to provide a lower impedance p'athto the highvfrequency currents .at the high frequency end of the tuning range thanth'e shaft provides; 7 I

The coupling fat thelow frequency end of the tuning range may then be established, regulated or controlled by the shaft size or. cross section throughout or. at spaced pointsalong the shaft an inductive winding and a variabletuning capacitor, a .control shaftjoining said capacitors and forming a commoninductive reactance providing conductive coupling between said circuits, independent adjustable capacity meansinshunt with each circuit for causing the shaft to conduct only a small part of the total circuit currents at the high frequency end of the tuning range of said circuits, and means for regulating the flow of coupling currents along said shaft. ,7 v

The combination with a plurality .of tunable hi h frequencycircuits, of means forelectrically co p11 ,g said circuits, means for tuning-said circuits including a variable capacitor in. each circuit, said capacitors each havinga stator section and arotorsection, means providing acomrnon conductive shaft connection between said rotor sections and providing a predetermined inductive reactance and a predetermined degree of con-' ductive coupling between said circuits through connection therewith only at the shaft ends, and independently adjustable means connected in shunt with each circuit while being exclusive of said shaft connection, and having capacitive reactance for modifying said degree of coupling in a predetermined high frequency portion of the tuning range, said shaft beingelectrically disco-ntinuous between rotor sections, and an impedance device conductively connecting the adjacent ends of at least one pair of shaft-sections.

3. The combination with a plurality of tunable high frequency circuits, of means for electrically coupling said circuits, meansfor tuning said circuits including a variable capacitor in each circuit, said capacitors each having a stator section and a rotor section, means providing a common conductive shaft connection between said rotor sections and providing a predetermined inductive reactance and a predetermined degree of conductive coupling between said circuits through connection therewith only at the shaft ends, and

independently adjustable means connected in shunt with each circuit while'being exclusive of said shaft connection, and having capacitive reactance for modifying said degree of coupling in a predetermined high frequency portion of the tuning range, said shaft connection comprising a plurality of conductive sections and a plurality of intercalated insulating devices, andan impedance device bridging at least one of said insulating devices, said impedance device thereby conductively connecting the adjacent 1 shaftsections.

4. The combination with a plurality of tunable high frequency circuits, of means for electrically coupling said circuits, means for tuning said circuits including a variable capacitor in each circuit, said capacitors each having a stator section and a rotor section, means providing a common conductive shaft connection between said rotor sections and providing a predetermined inductive reactance and a predetermined degree of conductive coupling between said circuits through connection therewith only at the shaft ends, and

independently adjustable means connected inshunt with each circuit while being exclusive of said shaft connection, and having capacitive reactance for modifying said degree of coupling in a predetermined high frequency portion of the tuning range, said shaft connection comprising a plurality of conductive sections and a plurality of intercalated insulating devices, and a resistance element conductively connecting at least two of said conductive sections.

5. In an ultra high frequency superheterodyne receiver, high frequency coupling means for oscillator, detector and signal input circuits comprising associated coupling inductors in each of said circuits and a variable tuning capacitor comprising a stator section and a trimmer capacitor connected with one side of each of said circuits, and a metallic frame providing a common connection for the opposite side of each of said circuits and the trimmer capacitors, said variable signal currents at the higher frequency end of the tuning range than the trimmer capacitors, whereby signal currents shift from the common coupling impedance of the shaft to the separate trimmer capacity paths across each circuit as the tuning range of operation is increased above a certain higher frequency in the tuning range. 6. In a high frequency radio signal receiving system,the combination of a plurality of tuned coupled signal circuits, each circuit comprising an inductive winding and a variable tuning capacitor, a control shaft joining said capacitors and forming a common inductive reactance providing conductive coupling between said circuits, independent adjustable capacity means in shunt with each circuit for causing the shaft to conduct only a small part of the total circuit currents at the high frequency end of the tuning range of said circuits, and variable means for controlling the flow of coupling currents along said shaft, said means including at least one section of the shaft between said capacitors of relatively small cross-sectional area, and a variable impedance device connected with the shaft in parallel relation to said section to provide a variable bridge connection therefor.

' 7; In a high frequency radio signal receiving system, the combination as defined in claim 1 further characterized by the fact that said variable tuning capacitors are included in a unit and that the inductive windings are mounted on said unit adjacent to said capacitors thereby to provide short low impedance lead connections for conducting said currents.

ALFRED H. TURNER 

